This continuation is built upon observations we have made in the past regarding the effects of physical training on the hearts of rats. All our previous studies have been performed in hearts of rats moderately trained by swimming. Therefore the responses we have described may apply only to a very specific model in which hypertrophy is absent. We are exploring whether the findings we have observed are also seen in rats subjected to other types of exercise. We are studying rats trained by running at moderate and at heavy work intensities (isotonic type exercise) and rats made to carry heavy loads up an incline (isometric type exercise). Male and female rats and pair fed males will be studied to investigate training that produces no hypertrophy, relative cardiac hypertrophy, and absolute cardiac hypertrophy. The effects of prior physical training on subsequent cardiac responses to ischemia and myocardial infarction will be employed. The effects of physical training on rats recovering from myocardial infarction will be investigated. The role thyroid activity may play in the development of a cardiac training effect will be studied. The possibility that adrenergic blocking agents prevent a cardiac training effect will be investigated. The studies to be performed on these various models will include measurements of papillary muscle function in the absence and presence of various pharmacologic agents in order to attempt to isolate mechanisms of excitation-contraction coupling; studies in the isolated working rat heart apparatus; and biochemical studies on contractile proteins, sarcoplasmic reticulum and sarcolemma. Finally the relation of the bradycardia effect of physical training to intrinsic cardiac mechanical responses will be investigated. Our long-term goals are not only to determine how physical training may promote better cardiovascular health, but we believe the various models of physical training proposed in the studies to be performed should provide considerable insight into general physiologic adaptive mechanisms of the heart.